The present invention relates to an optical amplifier having a doped fluoride glass optical fibre and process for the production of said amplifier. It has applications more particularly in the field of optical fibre telecommunications.
It is known that it is possible to use optical amplifiers with a fibre doped by a rare earth for producing telecommunications systems, both in transmission and in distribution.
An optical amplifier having a silica fibre doped by erbium has been the most widely studied.
For some time now studies have revealed that other glasses than silica could give rise to good quality optical fibres and said fibres could also be doped by rare earths.
Fluoride glass fibres, e.g. ZBLAN, are commercially available and it has been demonstrated that the doping of the core of such fibres by erbium (respectively praseo-dymium) made it possible to amplify a radiation having a wavelength close to 1.5 .mu.m (respectively 1.3 .mu.m).
For wavelengths close to 1.5 .mu.m, such erbium-doped fibres make it possible to produce an optical amplifier whose gain, as a function of the wavelength, is virtually constant in a natural manner, which is not the case for an erbium-doped silica fibre amplifier.
This property is of interest for transmitting a plurality of wavelengths and several methods have been proposed for "flattening" the gain spectrum of the amplifiers produced with the aid of doped silica fibres.
Thus, the use of erbium-doped fluoride glass fibres is the simplest solution for producing optical amplifiers with a flat spectral gain.
Moreover, recent results have shown that pumping efficiencies equivalent to the pumping efficiencies of silica doped by Er.sup.3+ ions could be obtained with such erbium-doped fluoride glass fibres.
For wavelengths close to 1.3 .mu.m, the only fibre optical amplifier which could appropriately operate at present is a praseodymium-doped fluoride glass fibre amplifier.
In telecommunications systems, these fluoride glass fibre-based amplifiers must be inserted between two ends of silica optical fibres and it is important that the input and output of said amplifiers are wired in standard silica optical fibres (CCITT recommendation G652). This gives rise to the problem of joining silica fibres and fluoride glass fibres.
It is not possible to fusion weld a doped or undoped, fluoride glass fibre with a silica fibre, as a result of the fact that the fusion temperatures differ widely. Therefore it is not possible to use methods for the production of silica-based optical amplifiers for producing a doped fluoride glass fibre optical amplifier.
For solving this problem, one known solution consists of joining a fluoride glass fibre and a standard silica fibre by means of a connector, e.g. of the OPTABALL (registered trademark) system marketed by Radial, whereof it is possible to adjust the dynamics losses.
It is therefore possible to produce an optical amplifier incorporating a doped fluoride glass optical fibre connected on one side to a silica fibre by means of such a connector (input connector) and on the other side to another silica fibre by a connector of the same type (output connector).
However, the reduction of the size of the optical mode in the doped fluoride glass fibre, necessary for having an optimum high pumping efficiency, leads to unacceptable losses on injecting the pumping radiation and the signal to be amplified, across the input connector, and at the extraction of the amplified signal, across the output connector of the doped fluoride glass fibre.
The adaption of the optical mode diameter between the standard silica fibre and the doped fluoride glass fibre can be improved by using an intermediate silica fibre having the same mode diameter as the doped fluoride glass fibre, said intermediate fibre being fusion welded to the standard fibre on one side and bonded to the doped fluoride glass fibre on the other side.
The fusion welding of the two silica fibres gives losses typically below 0.3 dB, whilst the bonding of the silica fibre to the doped fluoride glass fibre takes place with losses which are typically below 0.4 dB.
As a result of the optical losses induced by said bonding (due to the difference in the nature of the two glasses forming the fibres), the pumping efficiency of a doped fluoride glass fibre amplifier will still be below that of a comparable doped silica fibre amplifier. Moreover, the reliability of the bonding, which is traversed by a high optical power is not guaranteed.
The present invention obviates these disadvantages by proposing an optical amplifier having a doped fluoride glass optical fibre and a process for the production of said amplifier making it possible to both minimize the coupling losses of the doped fluoride glass fibre, prevent problems due to bonding, as indicated hereinbefore and have an optical amplifier input and output constituted by standard silica optical fibres.